For many years anthracite coal has been separated from mine tailings by forming a feedstock slurry composed of mine tailings and magnetite ore enriched heavy medium and flowing the slurry into cyclonic separators of the so-called tapered or constant acceleration type. The density of the heavy medium is controlled in relation to the specific gravity of the coal product to be produced. For example, depending upon the ash content, i.e. quality, of the desired coal product, the specific gravity of the heavy medium is usually maintained within about 0.2 units of the specific gravity at which separation is desired.
The quality of a coal product is related to its ash content. For instance, low ash coal, i.e. coal having an ash content of less than about 10 percent by weight, requires that separation occur at a specific gravity of about 1.75. Usually, this requires that the specific gravity of the magnetite ore enriched heavy medium be at least about 1.56.
U.S. Pat. No. 4,364,822, issued to the present Applicant, discloses an autogenous heavy medium process for separating low ash coal from refuse. In the patented process, a heavy medium is established by recirculating fine coal and refuse to build up the specific gravity of the medium to about 1.3 and subjecting a feedstock slurry which includes the heavy medium to a cyclonic separating action of a particular nature. The patented process provides the advantage of eliminating the need for expensive magnetite ore, reducing fresh water requirements and minimizing pumping costs.
While there is a high demand for low ash coal, there are applications where high ash coal can be used satisfactorily. For instance, in certain modern fluidized bed combustion equipment, high ash coal can be burned efficiently. Accordingly, for these applications, there is a demand for high ash coal, i.e. coal having an ash content above about 20 percent, or a specific gravity of at least about 1.85.
If an attempt were made to produce high ash coal utilizing conventional magnetite ore enriched cyclonic separating processes, the separating medium would have to be maintained at a specific gravity of at least about 1.75. Such a high specific gravity can be achieved by increasing the quantity of magnetite ore used, but this would also increase magnetite losses which must be controlled to produce coal efficiently. Furthermore, at these higher specific gravities, the viscosity of the magnetite ore enriched medium, and feedstock slurry, increases, thereby substantially increasing the energy required simply to pump the slurry. Moreover, at such high specific gravities, the viscosity of the heavy medium also increases, and this exacerbates the problem of separating coal efficiently.
The process disclosed in Applicant's aforementioned patent is not suitable for producing high ash coal, primarily because there is not enough fine material in the raw feedstock to create a sufficiently high non-magnetite heavy media. Also, the viscosity of the media would be too high.